Black or various other chromatic colors are developed on a titanium surface by varying the thickness of its oxide film coating the surface of metallic titanium or by forming its nitride film on its surface. For example, powdery metallic titanium which has been subjected to such a color development treatment is in use for pigments for coating and printing, colorants for fibers and ornaments, cosmetic and sintering materials and so on.
References available regarding blackening process of metallic titanium include: Method for Forming Black Coating on Titanium Surface by Dipping Metallic Titanium in Dilute Aqeous Solution of Fluoric Acid (Japanese Patent No. 1190252), Method for Blackening Copper Deposited on Titanium Surface (Gazette: Patent Publication No. Sho 58-23469) and Two Step Process Using Sulfuric Acid and Fluoric Acid (Proceedings: The 77-th Metal Surface Technology Association Symposium, p. 184), etc.
As regards color development methods for developing colors other than black, there are known gas phase process of forming oxide or nitride film of titanium on its surface by oxidation or nitration reaction in gases, anodic oxidation process of forming its oxide film on the titanium surface by energizing it in an aqueous solution with the metallic titanium as an anode, and the chemical oxidation process of forming its oxide film on the titanium surface by heating the metallic titanium in an inorganic acid.
Of these color development methods, the blackening process involves such problems as stripping of the black coating or the requirement of applying a heat treatment for reduction of brightness. Color development methods for developing colors other than black respectively involve following problems:
The gas phase process comprises a method of heating metallic titanium in an oxygen or nitrogen atmosphere, using an electric furnace etc. Since the titanium surface is tinted by the action of the interference of light from the oxide or nitride film grown on the titanium surface by the heating, the tone may be altered by the thickness of the film. This method is advantageous in that colors may be developed, irrespective of the configuration of material, e.g. lumpy, spongy, or powdery (spherical, scaly) or other shapes, but contrarily it has a drawback of scanty color variations. For example, when the oxide film is formed by the gas phase process, the bluey and browny color ranges are relatively wide, but the reddish and greenish ranges are narrow, and the pinky and greeny colors are impossible to develop. In the case of nitride film, as is well-known, the color variation is limited to golden colors. Moreover, the uniformity and reproducibility of color development are not good.
The anode oxidation process takes advantage of the phenomenon that an oxide film is formed on the titanium surface, when a DC current is passed at a constant current, using metallic titanium as the anode in an electrolytic cell. As a certain film tickness is reached, current does not flow, and the voltage and the film thickness are proportional to each other. Therefore, the tones are copious, reproducibility of each color is good, and the process is easy to control, but the black color is not obtainable. Besides, it has a fatal defect that its applicable configuration is limited to plate or lump shape. Further, since the tone is altered by fingerprints, and the film produced by this process has poor wear resistance, the film's durability quality is low.
The chemical oxidation process is a method for developing colors by harnessing the action of light interference, with an oxide film formed by subjecting metallic titanium to a boiling treatment in an inorganic acid. This method is simple, but its efficiency is low, taking long time in film growth. It provides only small color variations.
It is therefore an object of this invention to provide a color development method of metallic titanium which enables colored titanium rich in color variation, irrespective of the material configuration.
Another object of this invention is to provide a color development method of metallic titanium which enables manufacture of black titanium with low brightness, also black titanium with high close film adherence and other colored titanium.
Still another object of this invention is to provide black titanium and other colored titanium produced by any of these methods which are high in quality but low in price.